The invention provides a process for converting silicon tetrachloride to trichlorosilane.
Trichlorosilane (TCS) is used for the preparation of polycrystalline silicon.
TCS is typically prepared in a fluidized bed process from metallurgical silicon and hydrogen chloride. In order to obtain high-purity TCS, this is followed by a distillation. In this preparation, silicon tetrachloride (STC) is also obtained as a by-product.
The greatest amount of STC is obtained in the deposition of polycrystalline silicon.
Polycrystalline silicon is produced, for example, by means of the Siemens process. This involves depositing polycrystalline silicon in a reactor on heated thin rods. The process gas used as the silicon-containing component is a halosilane such as TCS, in the presence of hydrogen. The conversion of TCS (disproportionation) to deposited silicon forms large amounts of STC.
It is possible to produce fumed silica from STC, for example by reaction with hydrogen and oxygen at high temperatures in combustion chambers.
However, the use of STC that is of greatest economic interest is the conversion to TCS. This is effected by reaction of STC with hydrogen to give TCS and hydrogen chloride. This makes it possible to produce TCS again from the STC by-product formed in the deposition, and to feed that TCS back to the deposition operation in order to produce elemental silicon.
Two processes for conversion are known: the first process, called low-temperature conversion, is performed in the presence of one or more catalysts. However, the presence of catalysts (e.g. Cu) can adversely affect the purity of the TCS and hence of the silicon deposited therefrom. A second process, called high-temperature conversion, is an endothermic operation, wherein the formation of the products is equilibrium-limited. In order to arrive at any significant TCS production at all, very high temperatures have to be employed in the reactor (≧900° C.)
U.S. Pat. No. 3,933,985 A describes the reaction of STC with hydrogen to give TCS at temperatures in the range from 900° C. to 1200° C. and with a molar H2:SiCl4 ratio of 1:1 to 3:1. However, only yields of 12-13% are achieved.
For energy-saving reasons, the reactants of the reaction (STC and hydrogen) are often heated, typically with the aid of the hot offgases from the reactor (products and residues of the reactants, i.e. essentially TCS, hydrogen chloride, STC and hydrogen).
DE 30 24 320 C2 claims, for example, an apparatus for conversion of STC to TCS using a heat exchanger unit. The heat exchanger unit may consist, for example, of a set of electrically unheated graphite tubes which serve as a gas outlet for product gas, and reactant gas flows around the outside of these in countercurrent.
U.S. Pat. No. 4,217,334 A discloses a process for hydrogenation of STC with hydrogen to TCS within a temperature range of 900-1200° C. By virtue of a high molar H2:STC ratio (up to 50:1) and a liquid quench of the hot product gas below 300° C., distinctly higher TCS yields are achieved (up to about 35% at a molar H2:STC ratio=5:1). Disadvantages, however, are the distinctly higher hydrogen content in the reaction gas and the employment of a quench by means of a liquid, both of which greatly increase the energy expenditure in the process and hence the costs, especially since the cooling is effected without utilization of the energy released.
WO 2008/146741 A1 discusses the preparation of TCS by reduction of STC. The operation is divided into two reaction stages. The first stage is conducted within a first temperature range of 1000-1900° C. The first reaction stage is followed by cooling of the reaction gas to 950° C. or less within 1 s. In a second reaction step the temperature is kept at 600-950° C. for 0.01-5 s before cooling is effected to temperatures of less than 600° C.
U.S. Pat. No. 8,168,152 B2 likewise discloses a multistage cooling operation in the hydrogenation of STC to TCS. The reaction temperature is 1000-1900° C. Cooling is effected to a temperature of greater than or equal to 600° C. within 10 ms from the commencement of cooling, and to a temperature of less than or equal to 500° C. within 2 s. U.S. Pat. No. 8,168,152 B2 describes the necessity of a hold step in the cooling process, such that the temperature has to be kept at a temperature in the range of 500-950° C. over a period of 10-5000 ms, in order to decompose higher-order silanes which form and hence to prevent the formation of polymers.
EP 2 088 124 A1 discloses that high conversion rates are achieved by rapid cooling of a reaction gas mixture which is obtained by reaction of STC and H2 at temperatures of 900-1900° C. However, the high cooling rate is achieved by quenching to 800-300° C. Only at these relatively low temperatures is the energy released in the course of cooling transferred to the reactants.
EP 2 085 359 A1 describes a process in which STC and hydrogen are reacted at temperatures above 800° C. The product gas is cooled (quenched) to T less than or equal to 650° C. by means of a cooling gas within 1 s. High yields are obtained by quenching the reaction gas either by means of liquids or by means of gases. However, the energy removed in this context cannot be utilized in an economically viable manner.
DE 3024319 A1 likewise relates to a continuous process for preparing TCS by hydrogenation of STC in a high-temperature reactor at 900-1300° C. In this context, the reaction time in the reactor, however, is 200-2 s.
U.S. Pat. No. 8,197,784 B2 claims a process for preparing TCS, which is effected by reaction of STC- and H2-containing gases at supercritical pressure. In this case, the reactant gases reside in the reaction zone for 200-0.05 s and are cooled thereafter to 300° C. within 200-0.05 s.
US 2008/0112875 A1 discloses a process for preparing TCS by hydrogenation of STC at reaction temperatures of 700-1500° C., in which the product mixture is cooled to the cooling temperature (TCool) by means of a heat exchanger within a residence time of the reaction gases ofτ=A×exp(−B×TCool/1000)[ms](where A=4000; 6≦B≦50 and 100° C.≦TCool≦900° C.)the energy removed by means of a heat exchanger being used to heat the reactant gases. The residence times of the reaction gas in the reactor are τ≦0.5 s.
However, it has been found that, in the process according to US 2008/0112875 A1, there can be surprising operational losses in yield and hence in economic viability.
It was an object of the invention to avoid this.